In paper production, twin-wire forming and fourdrinier forming are two commonly used techniques. In twin-wire forming, the paper making machine comprises a headbox, a twin-wire nip, an inner and an outer moving wire, and a forming roll. The headbox is fitted with a lip for adjustment of the flow of pulp suspension. The pulp suspension exits the headbox as a jet and is directed into the nip. Thereafter, the suspension is contained between the inner and outer wires and is dewatered over the forming roll. In fourdrinier forming, the suspension exits the headbox as a jet and lands on a forming table comprising a single moving wire. Hybrid machines feature both a fourdrinier and a twin wire.
Throughout this application, the moving wire or wires containing the pulp suspension will be referred to as a moving paper web. The pulp suspension of the moving paper web exhibits relative movements within the web, the speed of which may vary and thus cause shifting thickness of the web. This means that the local velocity at a point in the web varies over the observed area. The expression “machine direction” refers to the direction parallel to the movement of the web, and “cross direction” refers to the direction perpendicular to the movement of the web.
It is known in the art to measure various properties of the moving paper web, such as formation characteristics (e.g. fiber flocs) or surface weight profile, or velocity averaged over the observed area. These properties can vary across the web, both in the cross direction and the machine direction. Variations in formation characteristics are a measure of the paper quality, whereas variations in velocity imply that the pulp is being stretched or fibers misaligned in a non-uniform manner, which will also affect the quality of the finished product.
Conventionally, such measurements are performed using one or more light sources directed at the paper web and one or more imaging devices (e.g. CCD cameras) positioned along the machine direction of the web, and capturing the light reflected off the web surface. The properties of interest can then be analyzed using image processing techniques.
The article “SOFA-STFI On-line Forming analyzer” (Söderberg L. D., 10th Control Systems Conference, Stockholm 2002) discloses a method for measuring properties in a moving paper web using transmitted rather than reflected light. The light source and the imaging device are positioned at opposite sides of the web, and the imaging device captures the light transmitted through the web. Measurements are performed in the machine direction only. A method for filtering out the wire pattern from the image is also disclosed.
Japanese patent application 2003-213585 discloses an apparatus for monitoring paper quality using transmitted light, where two or more cameras can be positioned crosswise over the web. This apparatus measures the average velocity of the observed area.
U.S. Pat. No. 6,118,132 discloses a system for measuring the velocity, displacement and strain on a moving surface or web of material. The system comprises two photo sensor arrays spaced a certain distance apart along the machine direction. Images from the two arrays are compared, and the displacement of a unique pattern on the web surface is measured and used to calculate the desired parameters. Reflected light is used. This apparatus measures the average velocity, displacement and strain in the observed area.
US 2006 0171 725 describes a two-dimensional detection system using an optical sensor to record images of the sheet, thereby measuring the velocity and direction of the sheet. This apparatus measures the average velocity, displacement and strain in the observed area.
Merely measuring the average velocity in accordance with the prior art cited above does not solve the problem of enabling local measurement and adjustment of the paper web in real time.
Therefore, a need still exists for more efficient and accurate methods for measuring the two-dimensional velocity of a moving paper web, for the sake of production efficiency and ensuring the quality of the end product.